COUNTERMANPAD Applications abound in which countermeasures are used to deflect or defeat an incoming shoulder-fired missile. In order to divert or defeat the incoming missile, infrared laser radiation is projected towards the target with a so-called jam code which confuses the seeker in the missile and causes the missile to be diverted away from its intended target.
Countermeasuring incoming missiles can be accomplished utilizing flares, and if the incoming missile is a heat-seeking missile, then the heat-seeking missile will follow all the flares as opposed to the original target.
However, if the target is large, meaning it has large bright engines which produce a significant IR signature, then the incoming missile will home in on the rather large thermal signature from the aircraft, such as a 747. If an IR countermeasure laser is used to countermeasure the missile and if its beam is too wide, there will not be enough energy on the missile's seeker and the missile will still home in on the bright engine. Thus, there is a requirement for a pencil thin laser beam to concentrate sufficiently high energy onto the missile's seeker.
If one had an infinitely high-power laser, one could tolerate a wide beam. However, this is not practical and in order to minimize laser power as well as size, it is more appropriate to generate a pencil-thin laser beam which concentrates its energy in a very small but intense spot. The intensity of this laser beam is such that it completely drowns out the thermal signature of a large target.
However, when generating a pencil-thin laser beam, pointing accuracy is of utmost importance. For instance, for such a pencil-thin laser beam there is a requirement that the pointing accuracy be less than a detector's Instantaneous-Field-of-View (IFOV), which at 8 miles is of less than 4 feet.
Ascertaining where the target is to this pointing accuracy is extremely difficult because the IR image of the target is blurred out on the focal plane array normally used and overlaps numbers of pixels. Normally, the resolution is limited to the IFOV of a pixel. For a Focal Plane Array (FPA), the angular error is associated with the size of a pixel, whereas the requirement is on the order of ¼ of a pixel's IFOV.
While auto boresighting techniques are utilized to establish the exact position of the laser beam in space, the center of energy of the blur is difficult to ascertain, especially when utilizing a focal point of array having a resolution limited by the pixel IFOV.
It is therefore important to be able to ascertain laser pointing direction and the IR image center to an accuracy better than that associated with the IFOV of a pixel.
As will be appreciated with a very tight pencil-thin laser beam used for concentrating energy in a very small cross-section, it is exceedingly important to be able to take the blurred out IR missile plume image which can overlap a number of pixels on the focal plane array and determine the exact point at which the laser beam is to be aimed to accuracies greater than the pixel level resolution of the array.
More particularly, detector arrays used in countermeasure systems employ a focal plane array onto which energy is imaged. The resolution of the focal plane array is determined by the size of the array and the pixel size. For very large arrays, such as 1028×1028 arrays, pixel size is very small and resolution is only limited by this small pixel size. In some cases pixel size can be under 10 microns and with improved optics the images can be focused down to such a small spot.
However, these large arrays are costly and exceed weight constraints. More importantly, the read out of so many pixels simultaneously or in rapid succession is difficult and requires a considerable amount of processing. There is therefore a need to provide the same resolution as a large array in a small affordable 128×128 array. To do this one needs sub-pixel resolution.
All things being equal, the size of the pixels determine the effective pixel IFOV and up until the present time, the resolution that determines the location of the target has been limited by the inability of small arrays to resolve focal plane array images below pixel level.
To put it another way, depending on how much jam energy one wants to put on the target, the efficiency of the jamming is related to how large the aircraft is, because the larger the aircraft, the brighter the engine. The brighter the engine, the more energy that must be focused onto the target to sufficiently increase the jam-to-signal ratio or J to S. Depending on the platform one uses, one has to generate a considerable J to S for bright engines.
One can either spread energy out into the environment like a big flashlight beam, but it is very weak compared to the bright engine. Thus, in order to project sufficient energy onto the target, one has to shrink the beam to pencil thinness, thus to concentrate the energy into a small spot. This means that track accuracy is very tight or the spot will miss the target.